1. Field of the Invention
The present invention relates to an epitaxial wafer and a production method thereof. More specifically, the present invention relates to a method for producing an epitaxial wafer comprising a step of growing a silicon crystal by the Czochralski method wherein hydrogen and nitrogen are added to a silicon melt, a step of preparing a silicon substrate by machining said silicon crystal, and a step of forming an epitaxial layer at the surface of the silicon substrate.
2. Background Art
With the progress of semiconductor devices in recent years, increasingly severe control of crystal quality of silicon wafers has been demanded. Especially, device characteristics, in particular Gate Oxide Integrity, can be deteriorated by defects such as grown-in defects existing in a silicon wafer produced by cutting a silicon crystal after being grown by the Czochralski method. These grown-in defects are oxygen precipitates and dislocations induced in the device production process, voids, or stacking faults. However, on the other hand, these aforementioned defects are utilized as gettering sites for heavy metals. As the concentration of the gettering sites decreases, the purity of the silicon wafer is compromised and the concentration of heavy metal impurities exceeds a level of 1×1010 atoms/cm2, which is the level required for the most advanced device production. Consequently, since it is necessary to reduce heavy metal impurities existing in a silicon wafer as far as is possible, controlling those defects is an extremely important task.
As one of the technologies for avoiding deterioration of device characteristics caused by voids at the surface of the silicon wafer, the use of epitaxial wafers is known, wherein the wafers are formed by deposition of an epitaxial layer by a CVD method on a surface of a mirror polished silicon substrate wafer obtained from a silicon single crystal. Because of the absence of voids in this epitaxial layer, a semiconductor wafer with good Gate Oxide Integrity can be attained. However, when the deposition of the epitaxial layer is performed, oxygen precipitates (Bulk micro defect: BMD) generated as a result of an oxygen deposit are not formed inside the epitaxial wafer while the silicon wafer is subjected to heat treatment in a device process. If no BMDs are generated, the gettering of heavy metals which contaminate the wafer during the device process cannot be done. Therefore, a problem of the deterioration of device characteristics results.
Accordingly, to solve such problems, T. Abe and H. Takeno, Mat. Res. Soc. Symp. Proc. Vol. 262, 3, 1992, for example, reports that by doping a silicon single crystal with nitrogen, the aggregation of crystal-lattice vacancies in silicon is suppressed, and that the crystal defect density is lowered. JP2000-109396 correlates to the technology described in the above-mentioned report, and proposes a method which comprises deposition of an epitaxial layer on a silicon substrate wafer obtained from a crystal with added nitrogen in order to sufficiently generate BMDs even after the deposition of the epitaxial layer. In this method, because nitrogen becomes a nucleus for the formation of BMDs, and because BMDs are generated even after the deposition of the epitaxial layer, an epitaxial wafer with superior gettering capability in heat treatment in device processing can be attained.
In addition, JP2008-150283 has proposed a method for reducing the number of defects by adding boron and by optimizing the thermal history of pulling and growing a silicon single crystal with a predetermined resistivity through the Czochralski method. Such method can reduce the number of epitaxial defects in the resultant wafers. Also, a higher in-plane uniformity of the density of oxygen precipitates as well as a higher gettering capability can be achieved. However, it has been clarified by recent studies that the thermally stable oxygen precipitates produced by nitrogen doping, which are difficult to eliminate even by heat treatment at high temperatures, cause a problem as they tend to induce defects in the epitaxial layer. Further, when the silicon is doped with boron, the gettering capability becomes lowered for heavy metal impurities unless a high concentration of boron is used.
Therefore, to solve the above problem, for example, JP3589119 has disclosed a technology for controlling the position where an OSF ring is generated depending on the pulling rate by controlling a gap L between the melt surface and the lower end of a thermal shielding material so as to adjust the position of the OSF ring to the pulling rate. Especially, there has been proposed a method for pushing the OSF region out of the crystal by setting the crystal pulling rate to a value equal to or higher than 1.2 mm/minute. It has been reported that, in this method, a density of the epitaxial layer defects can be reduced to a level equal to or lower than 0.1 cm−2 (defects per square centimeter) sufficiently suppressing the generation of the epitaxial layer defects.
In addition, as another method of technology to avoid the above-mentioned problem, for example, JP2008-100906 has proposed a method for doping with boron, nitrogen, and hydrogen. It is disclosed that in this method, without increasing the epitaxial layer defects on the epitaxial wafer, the gettering capability and a low density of epitaxial layer defects can be achieved by using hydrogen which weakens the effect of nitrogen in promoting the formation of oxygen-induced stacking faults.
Further, JP2000-281491 has proposed a method for growing a silicon single crystal by introducing hydrogen gas and incorporating nitrogen in the silicon melt at levels ranging from 1×1016 to 1.5×1019 cm−3 (atoms per cubic centimeter). Through this method, the generation of octahedron-like vacancy defects is suppressed to lower the size of the vacancy defects and to reduce the density of the oxygen precipitates in the single crystal.
However, as in JP2000-109396, when the silicon substrate wafer with added nitrogen is used as an epitaxial wafer substrate, nitrogen causes defects in the epitaxial layer. In addition, these nitrogen-induced defects in the epitaxial layer have been known to appear in the defect region called an OSF region of a nitrogen-doped substrate. One reason is that oxygen precipitates already existing in the OSF region at the stage just after crystal growth form new defects in the epitaxial layer. It should be noted that the term “OSF region”, as used herein, refers to a region where the concentration of the crystal lattice vacancies is extremely lowered because the crystal lattice vacancies and the interstitial silicon atoms recombine due to radial diffusion. Also, this is the region in which an oxidation heat treatment of the silicon wafer generates oxidation-induced stacking faults.
Doping the silicon wafer with boron at a high concentration, as in JP2008-150283 or JP2008-100906, causes a problem in that the boron impurities in the substrate diffuse out into the vapour phase during the deposition of the epitaxial layer and get incorporated into the epitaxial layer again. This phenomenon is called auto-doping. On the other hand, a silicon single crystal wafer doped with a low concentration of boron cannot solve the problem of the lower gettering capability for heavy metal impurities in segregation-type gettering by boron atoms, compared to the relaxation-type gettering using oxygen precipitates.
Further, as described in JP3589119, when the crystal size becomes equal to or larger than 300 mm, it is difficult to stably grow a crystal with a pulling rate equal to or higher than 1.2 mm/minute. In addition, it has been noted clearly that when an epitaxial layer is deposited after producing a silicon substrate wafer under such conditions as described in JP2000-281491, the epitaxial layer generates defects even when a substrate with no OSF region is used. It has been clarified that such epitaxial layer defect are generated from voids at the substrate surface (FIG. 1). As shown in FIG. 1, it has been confirmed that by doping the silicon crystal with nitrogen and hydrogen, voids are generated in the silicon crystal. Those voids which are located in the vicinity of the substrate surface before depositing the epitaxial layer may cause dislocations in the epitaxial layer. The dislocations are formed in the epitaxial layer at the surface of the substrate with the voids as an originating point.